Proppants for use in hydraulic fracturing of subterranean formations

ABSTRACT

A proppant for a well treatment fluid includes discrete particles of a substrate, such as sand, coated with a resin comprising a product of the Maillard reaction between a carbohydrate and an amine and/or an ammonium compound. Different resins, in particular thermoplastic or thermosetting resins, may be blended with Maillard reaction products or applied to the substrate as separate layers. The proppant may be included in a fracturing fluid, which is injected into a subterranean formation and used to stimulate hydrocarbon production from the subterranean formation.

CROSS-REFERNCE TO RELATED APPLICATION

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/087,709, filed on Apr. 15, 2011, which claims priority toU.S. Provisional Patent Application No. 61/345,207, filed on May 17,2010, which are both incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to proppants for use in hydraulic fracturing ofsubterranean formations.

BACKGROUND

Hydraulic fracturing is a term that has been applied to a variety ofmethods used to stimulate the production of fluids such as oil, naturalgas, from subterranean formations. In hydraulic fracturing, a fracturingfluid, usually an aqueous fluid, is injected through a well bore andagainst the face of the formation at a pressure and flow rate at leastsufficient to overcome the overburden pressure and to initiate and/orextend a fracture(s) into the formation. The fracturing fluid usuallycarries a proppant such as sand, bauxite, glass beads, etc., suspendedin the fracturing fluid and transported into a fracture. The proppantkeeps the formation from closing back down upon itself when the pressureis released. The proppant filled fractures provide permeable channelsthrough which the formation fluids can flow to the well bore andthereafter be withdrawn.

In hydraulic fracturing, proppant particles under high closure stresstend to fragment and disintegrate. For example, at closure stressesabove about 5000 psi (34,500 kPa), silica sand, the most commonproppant, is not normally employed due to its propensity todisintegrate. The resulting fines from this disintegration migrate andplug the interstitial flow passages in the propped formation. Thesemigratory fines drastically reduce the permeability of the proppedfracture.

Other propping agents have been used in an attempt to address thisproblem. Organic materials, such as the shells of walnuts, coconuts andpecans have been used with some success. These organic materials aredeformed rather than crushed when a fracture closes under the overburdenload. Aluminum propping agents are another type of propping agent thatdeform rather than fail under loading. While propping agents such asthese avoid the problem of creating fines, they suffer the infirmity ofallowing the propped fracture to close as the proppant is squeezedflatter and flatter with time. In addition, as these particles aresqueezed flat the spaces between the particles grow smaller. Thiscombination of decreased fracture width and decreased space between theparticles results in reduced flow capacities.

Another type of proppant includes spherical pellets of high strengthglass. These high strength glass proppants are vitreous, rigid and havea high compressive strength which allows them to withstand overburdenpressures of moderate magnitude. In addition, their uniform sphericalshape aids in placing the particles and providing maximum flow throughthe fracture. While these beads have a high strength when employed inmonolayers, they are less satisfactory in multilayer packs. In brine at250° F. (121° C.), the high strength glass beads have a tendency todisintegrate at stress levels between 5000 psi (34,500 kPa) and 6000 psi(41,400 kPa) with a resultant permeability which is no better, if notworse than sand under comparable conditions.

Resin coated particles have been used in efforts to improve thestability of proppants at high closure stresses. For example, U.S. Pat.No. 3,492,147 describes proppants useful in fracturing operations inwhich the proppants are produced by coating a particulate solid with aninfusible resin. The particulates to be coated include sand, nut shells,glass beads and aluminum pellets, whereas the resins used includeurea-aldehyde resins, phenol-aldehyde resins, epoxy resins, furfurylalcohol resins and polyester or alkyd resins.

Although resin coated particles have proven satisfactory in numerousapplications, concern exists over their use under high closure stresses.For example, some self-consolidating, resin-coated particles of theprior art do not develop their full strength until the resin coating hascured in the formation. In the event of rapid closure of the fracture,the proppant could be crushed before the resin has cured, resulting indecreased permeability. The use of dual resin coated particles asdescribed in U.S. Pat. No. 4,585,064 has therefore been proposed as away to alleviate this problem. In particular, in the '064 patent theproppant substrate is provided with an inner coating of a substantiallycured resin to increase the crush resistance of said substrate and anouter coating of a fusible curable resin which fuses and cures wheninjected into a formation to create a cohesive permeable mass.

One concern with the use of self-consolidating, resin-coated particlesis compatibility with the well treatment fluids used to transport suchparticles into a formation. To address this concern, U.S. Pat. No.5,837,656 discloses a dual resin coated proppant that combines the welltreatment fluid compatibility advantages of precured resin coatedparticles with the strength and resistance to migration ofself-consolidating proppants. The proppant comprises a particulatesubstrate coated with an inner coating of a fusible curable resin; andan outer coating of a substantially heat-cured resin, where the resin ofthe inner coating is selected from the group consisting ofphenol-aldehyde resins, urea-aldehyde resins, melamine-aldehyde resins,epoxy resins, furfuryl alcohol resins, and copolymers of such resins;the resin of the outer coating is selected from the group consisting ofphenol-aldehyde resins, urea-aldehyde resins, melamine-aldehyde resins,epoxy resins, furfuryl alcohol resins, and copolymers of such resins,and the resin of the outer coating is heat-curable at conditions thatleave the resin of the inner coating uncured.

Although resin coated particles offer significant advantages asproppants for well treatment fluids, the resins currently employed aregenerally derived from petroleum making them subject to the same supplyconstraints and price increases as their base raw material. In addition,many of the current resins, such as the phenolic resins, containimpurities, such as free phenol and/or free formaldehyde, which cannegatively interact with the fracturing fluid used to suspend the coatedproppant as it is being pumped into a formation. Furthermore, it hasbeen shown that non-reacted hexamethylenetetramine, which is commonlyused to cure or partially cure the novoalc phenolics, can also leach outand negatively impact the fracturing fluids as well.

U.S. Patent Application Publication No. 2008/0202750, published Aug. 28,2008, discloses thermoplastic coated proppants. These thermoplasticcoated proppants are both free-flowing and not tacky at ambientconditions. However, at elevated temperatures and pressures oftenencountered in subterranean formations, the coated proppants exhibitlatent tackiness which results in the agglomeration of the coatedproppants to form a stable framework of agglomerated proppant particles.Such a stable framework or network of agglomerated proppant particlesreduces both solid particle flow-back and the transport of formationfines from the subterranean formation. Examples of thermoplasticmaterials, which may be used to coat proppants, include polyethylene, apolypropylene, an ethylene vinyl acetate, an ethylene ethyl acrylate, astyrene-isoprene-styrene, an acrylonitrile-butadiene-styrene, astyrene-butadiene-styrene, a polystyrene, a polyurethane, an acrylicpolymer, a polyvinyl chloride, a fluoroplastic, a polysulfide, astyrene-acrylonitrile, a nylon, a phenol-formaldehyde novolac resin, orany combination thereof. In another aspect, the thermoplastic materialis a pine rosin, a modified rosin, a rosin ester, or any combinationthereof. Further examples of such thermoplastic materials include aterpene resin, a coumarone-indene resin, an oligomer of C₅ hydrocarbons,an oligomer of C₉ hydrocarbons, an oligomeric reaction product of aterpene and a phenolic, an oligomeric reaction product of a terpene anda styrenic, or combinations thereof. Generally, the number-averagemolecular weight of these oligomeric materials is less than about10,000, and more often, less than about 5000. The number-averagemolecular weight of the terpene resin, the coumarone-indene resin, theoligomer of C₅ hydrocarbons, the oligomer of C₉ hydrocarbons, theoligomeric reaction product of a terpene and a phenolic, and theoligomeric reaction product of a terpene and a styrenic, may be within arange from about 100 to about 4000. The number-average molecular weightof these materials may be in a range from about 125 to about 3000, fromabout 150 to about 2000, or from about 200 to about 1000.

According to the present invention, it has now been found that a resinsystem obtained as a product of the Maillard reaction between acarbohydrate and an amine or an ammonium compound provides an effectiveand advantageous coating for proppant particles. This invention may beused as a single layer to enhance crush resistance and especially whenused as the outer cured coating of multi-coated proppant particles, suchas described in U.S. Pat. No. 5,837,656. Thus, not only is the resinsystem derived from renewable biological resources, but also the systemdoes not contain the free phenol and/or free formaldehyde that candegrade many fracturing fluids.

U.S. Patent Application Publication No. 2007/0027283, published Feb. 1,2007, discloses a binder, comprising: Maillard reactants including (i)an amine and (ii) a carbohydrate, wherein the binder is (i) uncured and(ii) formaldehyde free. However, the binder is used to fabricatematerials from non or loosely assembled matter, such as glass orcellulose fibers. A similar binder for wood particles is disclosed inInternational Patent Publication No. WO 2008089847.

SUMMARY

A proppant for a well treatment fluid comprises discrete particles of asubstrate coated with a resin comprising a product of the Maillardreaction between a carbohydrate and an amine and/or an ammoniumcompound. The carbohydrate may be a monosaccharide, such as a reducingsugar. The reducing sugar may be at least one of dextrose, xylose,fructose, and dihydroxyacetone. The Maillard reaction may be between acarbohydrate and an ammonium salt of a carboxylic acid, which may be apolycarboxylic acid, such as citric acid.

The coating comprising the Maillard reaction product may be the onlycoating on the substrate. This coating may be composed entirely oressentially of the Maillard reaction product or it may be a blend of theMaillard reaction product with other polymenrs. The coating comprisingthe Maillard reaction product may be coated as a layer on the substrate.Other layers composed of a variety of materials may also be coated onthe substrate. These other layers may be underneath or above a layerformed by coating with the product of the Maillard reaction.

In one embodiment, a substrate is first coated with an innermost layerof a fusible resin and then with an outermost layer comprising a productof the Maillard reaction product. The fusible resin may be athermoplastic resin or a thermosetting resin.

Examples of thermoplastic resins are described in U.S. PatentApplication Publication No. 2008/0202750, published Aug. 28, 2008, andinclude an ethylene vinyl acetate, a phenol-formaldehyde novolac resin,a pine rosin, a modified rosin, a rosin ester, a terpene resin, acoumarone-indene resin, an oligomer of C₅ hydrocarbons, an oligomer ofC₉ hydrocarbons, an oligomeric reaction product of a terpene and aphenolic, and an oligomeric reaction product of a terpene and astyrenic, or any combination thereof

Examples of thermosetting resins are described in U.S. Pat. No.5,837,656 and include phenol-aldehyde resins, urea-aldehyde resins,melamine-aldehyde resins, epoxy resins, furfuryl alcohol resins, andcopolymers of such resins.

Examples of substrate particles include sand, glass beads, ceramics,carbon particles, bauxite, crushed nut (e.g., walnut or pecan) hulls,and other natural fibers, and man made polymeric particles, such asparticles of cured polyesters and phenolics.

The proppants may be included in a fracturing fluid, which is injectedinto a subterranean formation and used to stimulate hydrocarbonproduction from the subterranean formation.

DETAILED DESCRIPTION

Embodiments of the present invention combine the well treatment fluidcompatibility advantages of precured resin coated particle with thestrength and resistance to migration of self-consolidating proppants.This combination of properties may be achieved by creating a particlewith a thin cured resin coating that is compatible with fracturingfluids, yet still consolidates into a strong permeable mass whensubjected to downhole conditions.

Substrate

The present invention can be carried out with any suitable substrate.Choice of the particulate substrate is governed by the propertiesrequired by the particular application. One advantage of the inventionis that conventional frac sand can be rendered superior to the moreexpensive manufactured proppants, particularly in terms of their abilityto withstand crushing forces and to avoid subsequent fines generation.

For example, in the oil and gas industry extremely high strengthproppants are needed to hold open formation fractures created byhydraulic fracturing. In such an application, the present invention mayuse spherical glass beads as the particulate substrate. Such beads areavailable commercially in a variety of mesh sizes. For example, UnionCarbide Corporation supplies vitreous, rigid, inert, substantiallyspherical pellets under the trade name UCAR props. Such beads, while ofextremely high strength when employed in monolayers are lesssatisfactory when placed in multilayer packs. These beads when resincoated by the process of this invention and then cured in place yield apermeable mass of higher compressive strength than the beads alone.Beads from about 6 to about 200 mesh are generally used. In extremeenvironments where stresses are very high, sintered bauxite, aluminumoxide, and ceramics such as zirconium oxide and other mineralparticulates may be coated. Particles from 6 to 100 mesh are generallyused. (All reference to mesh size in the claims and specification are tothe U.S. standard sieve series).

Also suitable for use as substrates are various organic materials suchas walnut and pecan shells, synthetic polymers such as nylon,polyethylene and other resin particles. Metallic particles such as steeland aluminum pellets can also be coated.

Conventional frac sand is a preferred particulate substrate of theinvention. Silica sand of about 6 to 100 mesh (U.S. standard sieve) isgenerally used. One of the principal advantages of the instant inventionis that frac sand coated by the method of this invention is as strong orstronger than the more expensive proppants described above. Just asimportantly, in conditions where extreme stresses are expected theusable range of such high stress proppants as bauxite and the otherceramics can be extended by following the teachings of this invention.The utility of conventional frac sand can be extended into high stressapplications by first coating the sand with a cured coating. This resincoated sand may then be used as a substrate and at least one additionalresin coat may be applied as described below. Such coated particles mayexhibit the enhanced properties of dual-coated proppants described inU.S. Pat. No. 4,585,064, and the frac fluid compatibility of the instantinvention.

The type of sand used as a proppant substrate may come in a variety ofshapes and sizes. This sand may be relatively rounded or have sphericalgrains or the sand may be an angular sand having sharp or less roundedgrains. Similarly, particulate substrates other than sand, such asceramics, may be essentially spherical with rounded edges or angularwith sharp or jagged (not soft or rounded) edges.

Resins

In addition to Maillard reaction products, resins suitable for proppantcoatings include thermoplastic resins and thermosetting resins. Thesethermoplastic resins and thermosetting resins may be blended withMaillard reaction products or applied as separate coatings on asubstrate.

The thermoplastic resins may have latent tackiness, such that the coatedsubstrates are both free-flowing and not tacky at ambient conditions.However, at elevated temperatures and pressures often encountered insubterranean formations, the thermoplastic coated substrates exhibitlatent tackiness which results in the agglomeration of the coatedsubstrates to form a stable framework of agglomerated proppant. Examplesof thermoplastic resins are described in U.S. Patent ApplicationPublication No. 2008/0202750 and include pine rosins and chemicallymodified pine rosins.

Thermosetting resins are capable of being coated on the substrate andthen being cured to a higher degree of polymerization. Examples of suchthermosetting resins are described in U.S. Pat. No. 5,837,656 andinclude phenol-aldehyde resins of both the resole and novolac type,urea-aldehyde resins, melamine-aldehyde resins, epoxy resins andfurfuryl alcohol resins and copolymers of such resins. The resinspreferably form a solid non-tacky coating at ambient temperatures. Thisis desired so that the coated particles remain free flowing and so thatthey do not agglomerate under normal storage conditions.

The phenol-formaldehyde resins for coating substrates include truethermosetting phenolic resins of the resole type and phenolic novolacresins which may be rendered heat reactive by the addition of catalystand formaldehyde. Such resins may have softening points of 185° F. (85°C.) to 290° F. (143° C.).

A resin for coating a substrate may have both thermosetting andthermoplastic properties. Such resins may be prepared by blending one ormore suitable thermosetting resins with one or more thermoplasticresins. In such blends, the thermosetting resin may be in an uncured,partially cured or totally cured state.

A coupling agent may be incorporated during manufacture of the resinthat is to be used as a substrate coating. The coupling agent may haveat least one functional group to promote binding of the resin to thesubstrate, to promote binding of an outer coating layer to an innercoating layer, or to promote dispersion of a coated particle in a fluidto be injected into a formation to promote fracturing and propping. Thecoupling agent may be added in an amount ranging from about 0.1 to 10%by weight of the resin. A preferred range may be from about 0.1 to 3% byweight of the resin.

Substrates Coated with a Single Resin Coat

A substrate may be coated with a Maillard reaction product. The coatingallows the particle to withstand higher closure pressures beforecrushing and fines generation. Also the nature of the coating made fromthe Maillard reaction product contains no hexamethylenetetramine whichis often used to cure a phenolic coating. Thus, the cured Maillardreaction product coated particles exhibit less negative frac fluidinteractions and exemplifies the embodiment of this invention.

The substrate may be coated with Maillard reactants in an essentiallyuncured or partially cured state. The Maillard reactants would besufficiently solid and non-tacky at ambient conditions to provide acoated substrate, which is free flowing. However, at highertemperatures, such as those present in a subterranean formation, theMaillard reactants would fuse with Maillard reactants from adjacentcoated substrates and would intermingle and cure (e.g., withcrosslinking) to form an agglomerated mass of proppant.

Substrates Coated with Multiple Resin Coats

Substrates may be provided with two or more coatings. In one suchembodiment, the substrate may be first provided with an inner coating ofan uncured fusible (e.g., thermoplastic or thermosetting) resin and thenwith an outermost second coating of a resin formed from a Maillardreaction. After Maillard reactants are coated over the thermosettingresin, these Maillard reactants may be cured or partially cured toharden the outer coating. The temperature used in this curing stepshould be insufficient to promote curing of the thermosetting resin inthe inner layer. This dual coated substrate may be used in the mannerdescribed in the above-mentioned U.S. Pat. No. 5,837,656, whereby, dueto temperatures and pressures encountered in the formation, the resin ofthe inner coating flows through the crushed or fractured outer resinlayer to the exterior of the coated substrate and then cures to form anagglomerated mass of proppant particles in the formation.

In another embodiment of a dual coated substrate, the substrate is firstprovided with an inner coating of a thermoplastic resin and then with anoutermost second coating of a resin formed from a Maillard reaction.This dual coated substrate may be used in the manner described in theabove-mentioned U.S. Patent Application Publication No. 2008/0202750,whereby, due to temperatures and pressures encountered in the formation,the resin of the inner coating flows through the crushed or fracturedouter resin layer to the exterior of the coated substrate and binds withother coated substrates to form an agglomerated mass of proppantparticles in the formation.

Whenever a coating is applied to a substrate, the coating mayencapsulate individual substrate particles and have an essentiallyuniform thickness. However, the coating need not necessarily totallyencapsulate individual substrate particles, and the coatings need not beof uniform thickness. As long as the particles are coated to the extentthat the coating provides desirable properties, such as enhancedcompatibility with fracturing fluid, the particles need not be entirelyencapsulated with any coating material. For example, partially coatedparticles may be non-tacky and flowable under ambient conditions, yetretain latent tackiness, such that they form agglomerated masses ofparticles in the formation. The surface of such partially encapsulatedmay include bare spots, where part of the surface of the substrate isexposed. Coated substrates may also be combined with uncoated substratesor with substrates coated in a different manner.

The coated substrates may be discrete, that is, substantially notagglomerated under ambient conditions. However, small amounts, forexample, less than 5% by weight, based on the entire weight of thecoated particles, of agglomerated particles may be included, where, forexample, up to five substrate particles are bound together by a coatingmaterial. The weight of each coating on a substrate particle may be, forexample, from about 1 wt % to about 8 wt %, based on the entire weightof the coated particle. The coating thickness of each coating on asubstrate particle may be, on average, from about 0.1 to about 5 mils.

Inner Resin Coat

A preferred thermosetting resin of the inner coating to be used inembodiments described herein is a phenolic novolac resin. Particularlysuitable are phenolic novolac resins manufactured by Georgia PacificChemicals LLC, known as GP-2202 and GP-664G26. The GP-099N07 andGP-664G26 resins have a softening point range of 85-100° C. (185-212°F.). When either resin is used, it is necessary to add to the mixture across-linking agent to effect the subsequent curing of the resin.Hexamethylenetetramine is the preferred material for this function as itserves as both a catalyst and a source of formaldehyde, although othercross linking agents may be utilized as well, such as epoxies, andresoles.

A preferred thermoplastic resin of the inner coating in embodimentsdescribed herein is a pine rosin, a modified rosin, a rosin ester, orany combination thereof

Additives and process steps to minimize storage and handling problemshave been described. For example, U.S. Pat. No. 4,732,920, to Graham andSinclair, describes the addition of calcium stearate to preventsintering and mineral oil to prevent dust problems. Other additives maybe used.

A coupling agent may be employed. The choice of coupling agent isinfluenced by the resin to be used. For phenolic resins, the couplingagents include amino, epoxy, and ureido organo silanes. Epoxy modifiedgamma-glycidoxypropyltrimethoxysilane has given excellent results whenused in the amount of 0.50-1.00% based on the weight of the resin. Theuse of coupling agents as incorporated into the resin and as applieddirectly to the particulate substrate is discussed in Graham et al U.S.Pat. No. 4,518,039.

Outer Resin Coat

A Maillard reaction product may be used to form the outermost coating ofresin on the substrate. The outer coat of resin may be formed from aheat curable resin coating formed over a coating of as inner heatcurable resin. As stated previously, this outer resin may be curable atconditions that do not completely cure an inner curable coating.

As noted above, the outer coat of resin may also be entirely composed ofor partially composed of a thermoplastic resin.

Coating Process Parameters

The inner and outer resin coatings may be formed by a variety ofmethods. For example, the solvent coating process described in U.S. Pat.No. 3,929,191, to Graham et al., may be used. Other processes such asthat described in U.S. Pat. No. 3,492,147 to Young et al. describes thecoating of a particulate substrate with a liquid, uncatalyzed resincomposition characterized by its ability to extract a catalyst or curingagent from a non-aqueous solution. Phenol-formaldehyde novolac resinsfor the inner coat may be applied to the substrate using a hot meltcoating procedure. Such a procedure is described in U.S. Pat. No.4,585,064. A spray drying procedure may also be used to coat particles.

High strength particles of the invention may be manufactured in amulti-step process. In a first step, a curable resin, such as aphenol-formaldehyde resin or a thermoplastic, such as a pine oil resin,is coated on the particulate substrate. In the second step an outercoating is formed. When the inner resin is a thermosetting resin, theouter coating may be cured at conditions that leave the inner resincurable.

Formation of a Curable Inner Coating

The first or inner coating of resin may be formed on the particulatesubstrate by first coating the heated substrate with a thermoplasticphenol-formaldehyde novolac resin. This is followed by the addition of across linker such as hexamethylenetetramine in an amount (10 to 15%based on resin weight) which will convert the resin coating formthermoplastic to thermosetting. The subsequest coating additions aresuch that this first coating does not achieve full thermoset cure. Thiscoating may be carried out by preheating the particulate substrate to atemperature above the melting point of the particular resin used.

The particulate substrate may be heated to 350° to 500° F. (177° to 260°C.) prior to resin addition. The heated substrate may be charged to amixer or muller where from about 1% to about 6%, by weight of substrate,resin may be added. A preferred amount of resin based on the weight ofsubstrate is about 2% to 3%.

After completion of addition of the resin to the substrate, thesubstrate and melted resin are allowed to mix in the muller for a timesufficient to insure the formation of a uniform coating of resin on thesubstrate (e.g., sand), for example, from about 20 to about 45 seconds.

Following this mixing step from about 8% to about 15%, by weight of theresin, of a cross-linking agent, such as hexamethylenetetramine, may beadded to the substrate resin mixture. The amount ofhexamethylenetetramine may be about 13% by weight of the resin. Afteraddition of the hexamethylenetetramine the entire mixture may be allowedto mull for approximately 10 to 15 seconds. Then water may be added toquench the reaction of the inner resin coating. The amount of wateradded and the timing of its addition may be adjusted to quench thecuring of the inner resin while maintaining sufficient heat in theproppant to cure the outer coating that is added next. In place of thecooling water, a heat curable liquid resin, a resole for example, may beadded at this stage.

Formation of a Cured Outer Coating

A cured or substantially cured outer resin may then be coated over theinner resin. Substantially cured, as used herein, is to be interpretedas meaning that the cross-linking reaction of the resin is substantiallycomplete and that at typical downhole temperatures only minimaladditional curing takes place. The outer resin in a partially, yetsubstantially cured state, should provide a coating which substantiallycontains the inner coating and renders the coated substrate in a freeflowing state where the particles do not stick together prior to beingplaced downhole.

Precise control of the heat and mass balance is needed to ensure that acured outer coating layer sufficiently encapsulates a still curableinner resin coating. One skilled in the art will recognize that batchsize, equipment used, and resins and substrate selected will affectprocess conditions. Initial process temperature, process intervals,amounts of quench water added and amounts of solvent are allinterrelated and may be manipulated to arrive at an optimal process.Although experimentation may be required, optimization is within thelevel of skill in the art once the inventive concept is understood. Thistype of coating is exemplified in U.S. Pat. No. 5,837,656.

The Maillard Reaction

At least one coating on substrates described herein includes a productof a Maillard reaction. Maillard reactions produce melanoidins, i.e.,high molecular weight, furan ring and nitrogen-containing polymers thatvary in structure depending on the reactants and conditions of theirpreparation. Melanoidins display a C:N ratio, degree of unsaturation,and chemical aromaticity that increase with temperature and time ofheating. Accordingly, the subject coatings may be made via a Maillardreaction and thus contain melanoidins. It should be appreciated that thesubject coatings may contain melanoidins, or other Maillard reactionproducts. The melanoidins in the coating may be water-insoluble. TheMaillard reaction, Maillard reactants, melanoidins, and Maillardreaction products are described in the above-mentioned U.S. PatentApplication Publication No. 2007/0027283.

The Maillard reactants to produce a melanoidin may include an aminereactant reacted with a reducing-sugar carbohydrate reactant. Forexample, in one variation, an ammonium salt of a carboxylic acid may bereacted with (i) a monosaccharide in its aldose or ketose form or (ii) apolysaccharide or (iii) with combinations thereof. The carboxylic acidmay be, for example, a monomeric polycarboxylic acid, and the reactionmixture may, optionally, include a polyol. In another particularvariation, an ammonium salt of a polymeric polycarboxylic acid may becontacted with (i) a monosaccharide in its aldose or ketose form or (ii)a polysaccharide, or (iii) with combinations thereof. In yet anotherparticular variation, an amino acid may be contacted with (i) amonosaccharide in its aldose or ketose form, or (ii) with apolysaccharide or (iii) with combinations thereof. Furthermore, apeptide may be contacted with (i) a monosaccharide in its aldose orketose form or (ii) with a polysaccharide or (iii) with combinationsthereof. Moreover, a protein may be contacted with (i) a monosaccharidein its aldose or ketose form or (ii) with a polysaccharide or (iii) withcombinations thereof. The Maillard reactants may contain protein orpeptide or these reactants may be free of protein or peptide.

An outer coating on a substrate may include melanoidins produced innon-sugar variants of Maillard reactions. In these reactions an aminereactant is contacted with a non-carbohydrate carbonyl reactant. In oneillustrative variation, an ammonium salt of a monomeric carboxylic acidis contacted with a non-carbohydrate carbonyl reactant such as,pyruvaldehyde, acetaldehyde, crotonaldehyde, 2-furaldehyde, quinone,ascorbic acid, or the like, or with combinations thereof. In anothervariation, an ammonium salt of a polymeric polycarboxylic acid may becontacted with a non-carbohydrate carbonyl reactant such as,pyruvaldehyde, acetaldehyde, crotonaldehyde, 2-furaldehyde, quinone,ascorbic acid, or the like, or with combinations thereof. In yet anotherillustrative variation, an amino acid may be contacted with anon-carbohydrate carbonyl reactant such as, pyruvaldehyde, acetaldehyde,crotonaldehyde, 2-furaldehyde, quinone, ascorbic acid, or the like, orwith combinations thereof. In another illustrative variation, a peptidemay be contacted with a non-carbohydrate carbonyl reactant such as,pyruvaldehyde, acetaldehyde, crotonaldehyde, 2-furaldehyde, quinone,ascorbic acid, or the like, or with combinations thereof. In stillanother illustrative variation, a protein may contacted with anon-carbohydrate carbonyl reactant such as, pyruvaldehyde, acetaldehyde,crotonaldehyde, 2-furaldehyde, quinone, ascorbic acid, and the like, orwith combinations thereof

The melanoidins discussed herein may be generated from melanoidinreactant compounds. These reactant compounds may be disposed in anaqueous solution at an alkaline pH and therefore may not be corrosive.That is, the alkaline solution prevents or inhibits the eating orwearing away of a substance, such as metal, caused by chemicaldecomposition brought about by, for example, an acid. The reactantcompounds may include a reducing-sugar carbohydrate reactant and anamine reactant. In addition, the reactant compounds may include anon-carbohydrate carbonyl reactant and an amine reactant.

The Maillard reactants may be applied in an uncured or partially curedstate to a substrate, which is uncoated or has been precoated with acurable (i.e. thermosetting) or thermoplastic inner coating. Forexample, the Maillard reactants may be combined at room temperature in asolvent and maintained at room temperature with mixing until thereactants are properly mixed. The liquid mixture of reactants andsolvent may then be applied to a substrate precoated with a curableinner coating, followed by heating to induce substantial curing of theouter coating, without inducing substantial curing of the inner coating.

When Maillard reactants are cured they undergo chemical change. Examplesof these chemical changes include, but are not limited to, (i) covalentbonding, (ii) hydrogen bonding of coating components, and chemicalcross-linking of polymers and/or oligomers in the coating. These changesmay increase the coating's durability and solvent resistance as comparedto the uncured coating. Curing a coating may result in the formation ofa thermoset material. Furthermore, curing may include the generation ofmelanoidins. Curing can be initiated by, for example, heat,electromagnetic radiation, or electron beams.

As used herein, the term “alkaline” indicates a solution having a pHthat is greater than or equal to about 7. For example, the pH of thesolution can be less than or equal to about 10. In addition, thesolution may have a pH from about 7 to about 10, or from about 8 toabout 10, or from about 9 to about 10.

As used herein, the term “ammonium” includes, but is not limited to,⁺NH₄, ⁺NH₃R¹ and ⁺NH₂R¹R², where R¹ and R² are each independentlyselected in ⁺NH₂R¹R², and where R¹ and R² are selected from alkyl,cycloalkyl, alkenyl, cycloalkenyl, heterocyclyl, aryl, and heteroaryl.

The term “alkyl” refers to a saturated monovalent chain of carbon atoms,which may be optionally branched; the term “cycloalkyl” refers to amonovalent chain of carbon atoms, a portion of which forms a ring; theterm “alkenyl” refers to an unsaturated monovalent chain of carbon atomsincluding at least one double bond, which may be optionally branched;the term “cycloalkenyl” refers to an unsaturated monovalent chain ofcarbon atoms, a portion of which forms a ring; the term “heterocyclyl”refers to a monovalent chain of carbon and heteroatoms, wherein theheteroatoms are selected from nitrogen, oxygen, and sulfur, a portion ofwhich, including at least one heteroatom, form a ring; the term “aryl”refers to an aromatic mono or polycyclic ring of carbon atoms, such asphenyl, naphthyl, and the like; and the term “heteroaryl” refers to anaromatic mono or polycyclic ring of carbon atoms and at least oneheteroatom selected from nitrogen, oxygen, and sulfur, such aspyridinyl, pyrimidinyl, indolyl, benzoxazolyl, and the like. It is to beunderstood that each of alkyl, cycloalkyl, alkenyl, cycloalkenyl, andheterocyclyl may be optionally substituted with independently selectedgroups such as alkyl, haloalkyl, hydroxyalkyl, aminoalkyl, carboxylicacid and derivatives thereof, including esters, amides, and nitriles,hydroxy, alkoxy, acyloxy, amino, alkyl and dialkylamino, acylamino,thio, and the like, and combinations thereof. It is further to beunderstood that each of aryl and heteroaryl may be optionallysubstituted with one or more independently selected substituents, suchas halo, hydroxy, amino, alkyl or dialkylamino, alkoxy, alkylsulfonyl,cyano, nitro, and the like.

As used herein, the term “carboxylic acid” indicates an organic acidwith one or more carboxyl groups (—COOH). Carboxyl groups (—COOH) arealso referred to herein as carboxylic acid groups (—COOH). A carboxylicacid may be one or more of a monomeric monocarboxylic acid, a monomericpolycarboxylic acid, a polymeric polycarboxylic acid, an anhydridethereof, or a combination thereof. In one aspect, the carboxylic acidammonium salt reactant is sufficiently non-volatile to maximize itsability to remain available for reaction with the carbohydrate reactantof a Maillard reaction. In another aspect, the carboxylic acid ammoniumsalt reactant may be substituted with other chemical functional groups.These functional groups may promote, for example, one or more of thefollowing properties: (1) bonding with substrate particles; (2)compatibility with inner or outer coatings; (3) miscibility with theliquid phase of a fracturing fluid; (4) crosslinking with a suitablecrosslinking agent; and (5) bonding with fractured walls of a fracturedformation to which proppant particles have been introduced.

As used herein, the term “polycarboxylic acid” indicates a dicarboxylic,tricarboxylic, tetracarboxylic, pentacarboxylic, and like monomericpolycarboxylic acids, and anhydrides, and combinations thereof, as wellas polymeric polycarboxylic acids, anhydrides, copolymers, andcombinations thereof.

Illustratively, a monomeric polycarboxylic acid may be a dicarboxylicacid, including, but not limited to, unsaturated aliphatic dicarboxylicacids, saturated aliphatic dicarboxylic acids, aromatic dicarboxylicacids, unsaturated cyclic dicarboxylic acids, saturated cyclicdicarboxylic acids, hydroxy-substituted derivatives thereof, and thelike. Or, illustratively, the polycarboxylic acid(s) itself may be atricarboxylic acid, including, but not limited to, unsaturated aliphatictricarboxylic acids, saturated aliphatic tricarboxylic acids, aromatictricarboxylic acids, unsaturated cyclic tricarboxylic acids, saturatedcyclic tricarboxylic acids, hydroxy-substituted derivatives thereof, andthe like. It is appreciated that any such polycarboxylic acids may beoptionally substituted, such as with hydroxy, halo, alkyl, alkoxy, andthe like. In one variation, the polycarboxylic acid is the saturatedaliphatic tricarboxylic acid, citric acid. Other suitable polycarboxylicacids are contemplated to include, but are not limited to, aconiticacid, adipic acid, azelaic acid, butane tetracarboxylic acid dihydride,butane tricarboxylic acid, chlorendic acid, citraconic acid,dicyclopentadiene-maleic acid adducts, diethylenetriamine pentaaceticacid, adducts of dipentene and maleic acid, ethylenediamine tetraaceticacid (EDTA), fully maleated rosin, maleated tall-oil fatty acids,fumaric acid, glutaric acid, isophthalic acid, itaconic acid, maleatedrosin oxidized with potassium peroxide to alcohol then carboxylic acid,maleic acid, malic acid, mesaconic acid, biphenol A or bisphenol Freacted via the KOLBE-Schmidt reaction with carbon dioxide to introduce3-4 carboxyl groups, oxalic acid, phthalic acid, sebacic acid, succinicacid, tartaric acid, terephthalic acid, tetrabromophthalic acid,tetrachlorophthalic acid, tetrahydrophthalic acid, trimellitic acid,trimesic acid, and the like, and anhydrides, and combinations thereof.

Illustratively, a polymeric polycarboxylic acid may be an acid, forexample, polyacrylic acid, polymethacrylic acid, polymaleic acid, andlike polymeric polycarboxylic acids, copolymers thereof, anhydridesthereof, and mixtures thereof. Examples of commercially availablepolyacrylic acids include AQUASET-529 (Rohm & Haas, Philadelphia, Pa.,USA), CRITERION 2000 (Kemira, Helsinki, Finland, Europe), and SOKALAN(BASF, Ludwigshafen, Germany, Europe). With respect to SOKALAN, this isa water-soluble polyacrylic copolymer of acrylic acid and maleic acid,having a molecular weight of approximately 4000. AQUASET-529 is acomposition containing polyacrylic acid cross-linked with glycerol, alsocontaining sodium hypophosphite as a catalyst. CRITERION 2000 is anacidic solution of a partial salt of polyacrylic acid, having amolecular weight of approximately 2000.

Further, compositions including polymeric polycarboxylic acids are alsocontemplated to be useful in preparing the coatings described herein,such as those compositions described in U.S. Pat. Nos. 5,318,990,5,661,213, 6,136,916, and 6,331,350. In particular, in U.S. Pat. Nos.5,318,990 and 6,331,350 an aqueous solution of a polymericpolycarboxylic acid, a polyol, and a catalyst is described. A Maillardreaction to form a coating may take place simultaneously with a reactionto form a polymeric ester from a polycarboxylic acid and a polyol, asdescribed in U.S. Pat. Nos. 5,318,990 and 6,331,350. In such asimultaneous reaction, a blend of Maillard reaction products withpolymeric esters may form, in situ, during the formation of the coating.

As described in U.S. Pat. Nos. 5,318,990 and 6,331,350, the polymericpolycarboxylic acid comprises an organic polymer or oligomer containingmore than one pendant carboxyl group. The polymeric polycarboxylic acidmay be a homopolymer or copolymer prepared from unsaturated carboxylicacids including, but not necessarily limited to, acrylic acid,methacrylic acid, crotonic acid, isocrotonic acid, maleic acid, cinnamicacid, 2-methylmaleic acid, itaconic acid, 2-methylitaconic acid,α,β-methyleneglutaric acid, and the like. Alternatively, the polymericpolycarboxylic acid may be prepared from unsaturated anhydridesincluding, but not necessarily limited to, maleic anhydride, itaconicanhydride, acrylic anhydride, methacrylic anhydride, and the like, aswell as mixtures thereof. Methods for polymerizing these acids andanhydrides are well-known in the chemical art. The polymericpolycarboxylic acid may additionally comprise a copolymer of one or moreof the aforementioned unsaturated carboxylic acids or anhydrides and oneor more vinyl compounds including, but not necessarily limited to,styrene, α-methylstyrene, acrylonitrile, methacrylonitrile, methylacrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, methylmethacrylate, n-butyl methacrylate, isobutyl methacrylate, glycidylmethacrylate, vinyl methyl ether, vinyl acetate, and the like. Methodsfor preparing these copolymers are well-known in the art. The polymericpolycarboxylic acids may comprise homopolymers and copolymers ofpolyacrylic acid. The molecular weight of the polymeric polycarboxylicacid, and in particular polyacrylic acid polymer, may be is less than10000, less than 5000, or about 3000 or less. For example, the molecularweight may be 2000.

Compositions including polymeric polycarboxylic acids described in U.S.Pat. Nos. 5,661,213 and 6,136,916 that are contemplated to be useful inpreparing the coatings described herein comprise an aqueous solution ofa polymeric polycarboxylic acid, a polyol containing at least twohydroxyl groups, and a phosphorous-containing accelerator, wherein theratio of the number of equivalents of carboxylic acid groups, to thenumber of equivalents of hydroxyl groups is from about 1:0.01 to about1:3.

As disclosed in U.S. Pat. Nos. 5,661,213 and 6,136,916, the polymericpolycarboxylic acid may be, a polyester containing at least twocarboxylic acid groups or an addition polymer or oligomer containing atleast two copolymerized carboxylic acid-functional monomers. Thepolymeric polycarboxylic acid may be an addition polymer formed from atleast one ethylenically unsaturated monomer. The addition polymer may bein the form of a solution of the addition polymer in an aqueous mediumsuch as, an alkali-soluble resin which has been solubilized in a basicmedium; in the form of an aqueous dispersion, for example, anemulsion-polymerized dispersion; or in the form of an aqueoussuspension. The addition polymer must contain at least two carboxylicacid groups, anhydride groups, or salts thereof. Ethylenicallyunsaturated carboxylic acids such as, methacrylic acid, acrylic acid,crotonic acid, fumaric acid, maleic acid, 2-methyl maleic acid, itaconicacid, 2-methyl itaconic acid, α,β-methylene glutaric acid, monoalkylmaleates, and monoalkyl fumarates; ethylenically unsaturated anhydrides,for example, maleic anhydride, itaconic anhydride, acrylic anhydride,and methacrylic anhydride; and salts thereof, at a level of from about1% to 100%, by weight, based on the weight of the addition polymer, maybe used. Additional ethylenically unsaturated monomer may includeacrylic ester monomers including methyl acrylate, ethyl acrylate, butylacrylate, 2-ethylhexyl acrylate, decyl acrylate, methyl methacrylate,butyl methacrylate, isodecyl methacrylate, hydroxyethyl acrylate,hydroxyethyl methacrylate, and hydroxypropyl methacrylate; acrylamide orsubstituted acrylamides; styrene or substituted styrenes; butadiene;vinyl acetate or other vinyl esters; acrylonitrile or methacrylonitrile;and the like. The addition polymer containing at least two carboxylicacid groups, anhydride groups, or salts thereof may have a molecularweight from about 300 to about 10,000,000. A molecular weight from about1000 to about 250,000 may be used. When the addition polymer is analkali-soluble resin having a carboxylic acid, anhydride, or saltthereof, content of from about 5% to about 30%, by weight based on thetotal weight of the addition polymer, a molecular weight from about10,000 to about 100,000 may be utilized. Methods for preparing theseadditional polymers are well-known in the art.

As described in U.S. Pat. Nos. 5,661,213 and 6,136,916, the polyol (in acomposition including a polymeric polycarboxylic acid) contains at leasttwo hydroxyl groups and should be sufficiently nonvolatile that itremains substantially available for reaction with the polymericpolycarboxylic acid in the composition during heating and curingoperations. The polyol may be a compound with a molecular weight lessthan about 1000 bearing at least two hydroxyl groups, for example,ethylene glycol, glycerol, pentaerythritol, trimethylol propane,sorbitol, sucrose, glucose, resorcinol, catechol, pyrogallol,glycollated ureas, 1,4-cyclohexane diol, diethanolamine,triethanolamine, and certain reactive polyols, for example,β-hydroxyalkylamides, for example,bis-[N,N-di(β-hydroxyethyl)]adipamide,bis[N,N-di(β-hydroxypropyl)]azelamide,bis[N—N-di(β-hydroxypropyl)]adipamide,bis[N—N-di(β-hydroxypropyl)]glutaramide,bis[N—N-di(β-hydroxypropyl)]succinamide, andbis[N-methyl-N-(β-hydroxyethyl)]oxamide, or it may be an additionpolymer containing at least two hydroxyl groups such as, polyvinylalcohol, partially hydrolyzed polyvinyl acetate, and homopolymers orcopolymers of hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate,and the like.

Phosphorous-containing accelerators are described, for example in U.S.Pat. Nos. 5,661,213 and 6,136,916. The phosphorous-containingaccelerator (in a reaction mixture including a polymeric polycarboxylicacid and a polyol) may be a compound with a molecular weight less thanabout 1000 such as, an alkali metal hypophosphite salt, an alkali metalphosphite, an alkali metal polyphosphate, an alkali metal dihydrogenphosphate, a polyphosphoric acid, and an alkyl phosphinic acid or it maybe an oligomer or polymer bearing phosphorous-containing groups such as,addition polymers of acrylic and/or maleic acids formed in the presenceof sodium hypophosphite, addition polymers prepared from ethylenicallyunsaturated monomers in the presence of phosphorous salt chain transferagents or terminators, and addition polymers containing acid-functionalmonomer residues such as, copolymerized phosphoethyl methacrylate, andlike phosphonic acid esters, and copolymerized vinyl sulfonic acidmonomers, and their salts. The phosphorous-containing accelerator may beused at a level of from about 1% to about 40%, by weight based on thecombined weight of the carboxyoic acid and the polyol. A level ofphosphorous-containing accelerator of from about 2.5% to about 10%, byweight based on the combined weight of the carboxylic acid and thepolyol, may be utilized.

As used herein, the term “amine base” includes, but is not limited to,ammonia, a primary amine, i.e., NH₂R¹, and a secondary amine, i.e.,NHR¹R², where R¹ and R² are each independently selected in NHR¹R², andwhere R¹ and R² are selected from alkyl, cycloalkyl, alkenyl,cycloalkenyl, heterocyclyl, aryl, and heteroaryl, as defined herein.Illustratively, the amine base may be substantially volatile orsubstantially non-volatile under conditions sufficient to promoteformation of the thermoset coating during thermal curing.Illustratively, the amine base may be a substantially volatile base,such as, ammonia, ethylamine, diethylamine, dimethylamine, andethylpropylamine. Alternatively, the amine base may be a substantiallynon-volatile base, for example, aniline, 1-naphthylamine,2-naphthylamine, and para-aminophenol.

As used herein, “reducing sugar” indicates one or more sugars thatcontain aldehyde groups, or that can tautomerize, to contain aldehydegroups, which groups are reactive with an amino group under Maillardreaction conditions and which groups may be oxidized with, for example,Cu⁺² to afford carboxylic acids. It is also appreciated that any suchcarbohydrate reactant may be optionally substituted, such as withhydroxy, halo, alkyl, alkoxy, and the like. It is further appreciatedthat in any such carbohydrate reactant, one or more chiral centers arepresent, and that both possible optical isomers at each chiral centerare contemplated to be included in the invention described herein.Further, it is also to be understood that various mixtures, includingracemic mixtures, or other diastereomeric mixtures of the variousoptical isomers of any such carbohydrate reactant, as well as variousgeometric isomers thereof, may be used in one or more embodimentsdescribed herein.

Examples of amine reactants for a Maillard reaction include proteins,peptides, amino acids, ammonium salts of carboxylic acids, ammoniumsalts of polymeric polycarboxylic acids, and ammonium salts of monomericpolycarboxylic acids. In this context, “ammonium” can be [⁺NH₄]_(x),[³⁰NH₃R¹]_(x), and [³⁰NH₃R¹R²]_(x), where x is at least about 1. Withrespect to ⁺NH₂R¹R², R¹ and R² are each independently selected.Moreover, R¹ and R² are selected from alkyl, cycloalkyl, alkenyl,cycloalkenyl, heterocyclyl, aryl, and heteroaryl, as described above.Examples of reducing-sugar reactants for producing melanoidins,including monosaccharides, in their aldose or ketose form,polysaccharides, or combinations thereof. Illustrative non-carbohydratecarbonyl reactants for producing melanoidins include various aldehydes,e.g., pyruvaldehyde and furfural, as well as compounds such as ascorbicacid and quinone.

A Maillard reaction culminates in the production of melanoidins. In itsinitial phase, a Maillard reaction involves a carbohydrate reactant, forexample, a reducing sugar (note that the carbohydrate reactant may comefrom a substance capable of producing a reducing sugar under Maillardreaction conditions). The reaction also involves condensing thecarbohydrate reactant (e.g., reducing sugar) with an amine reactant,i.e., a compound possessing an amino group. In other words, thecarbohydrate reactant and the amine reactant are the melanoidinreactants for a Maillard reaction. The condensation of these twoconstituents produces an N-substituted glycosylamine. The compoundpossessing a free amino group in a Maillard reaction may be present inthe form of an amino acid. The free amino group can also come from aprotein or a peptide where the free amino groups are available in theform of, for example, the ε-amino group of lysine residues, and/or theα-amino group of the terminal amino acid.

Another aspect of conducting a Maillard reaction as described herein isthat, initially, the aqueous Maillard reactant solution (which also is acoating), as described above, may have an alkaline pH. However, oncecuring is initiated, the pH decreases (e.g., the coating may becomeslightly acidic).

The following discussion is directed to (i) examples of carbohydrate andamine reactants, which can be used in a Maillard reaction and (ii) howthese reactants can be combined. First, it should be understood that anycarbohydrate and/or compound possessing a primary or secondary aminogroup, which will act as a reactant in a Maillard reaction, can beutilized in the coatings of the present invention. Such compounds can beidentified and utilized by one of ordinary skill in the art with theguidelines disclosed herein.

With respect to exemplary reactants, it should also be appreciated thatusing an ammonium salt of a carboxylic acid as an amine reactant is aneffective reactant in a Maillard reaction. Ammonium salts of carboxylicacids can be generated by neutralizing the acid groups with an aminebase, thereby producing carboxylic acid ammonium salt groups. Completeneutralization, i.e., about 100% calculated on an equivalents basis, mayeliminate any need to titrate or partially neutralize acid groups in thecarboxylic acid(s) prior to coating formation. However, it is expectedthat less-than-complete neutralization would not inhibit formation ofthe coating. Note that neutralization of the acid groups of thecarboxylic acid(s) may be carried out either before or after thepolycarboxylic acid(s) is mixed with the carbohydrate(s).

With respect to the carbohydrate reactant, it may include one or morereactants having one or more reducing sugars. In one aspect, anycarbohydrate reactant should be sufficiently nonvolatile to maximize itsability to remain available for reaction with the carboxylic acidammonium salt reactant. The carbohydrate reactant may be amonosaccharide in its aldose or ketose form, including a triose, atetrose, a pentose, a hexose, or a heptose; or a polysaccharide; orcombinations thereof. A carbohydrate reactant may be a reducing sugar,or one that yields one or more reducing sugars in situ under thermalcuring conditions. For example, when a triose serves as the carbohydratereactant, or is used in combination with other reducing sugars and/or apolysaccharide, an aldotriose sugar or a ketotriose sugar may beutilized, such as glyceraldehyde and dihydroxyacetone, respectively.When a tetrose serves as the carbohydrate reactant, or is used incombination with other reducing sugars and/or a polysaccharide,aldotetrose sugars, such as erythrose and threose; and ketotetrosesugars, such as erythrulose, may be utilized. When a pentose serves asthe carbohydrate reactant, or is used in combination with other reducingsugars and/or a polysaccharide, aldopentose sugars, such as ribose,arabinose, xylose, and lyxose; and ketopentose sugars, such as ribulose,arabulose, xylulose, and lyxulose, may be utilized. When a hexose servesas the carbohydrate reactant, or is used in combination with otherreducing sugars and/or a polysaccharide, aldohexose sugars, such asglucose (i.e., dextrose), mannose, galactose, allose, altrose, talose,gulose, and idose; and ketohexose sugars, such as fructose, psicose,sorbose and tagatose, may be utilized. When a heptose serves as thecarbohydrate reactant, or is used in combination with other reducingsugars and/or a polysaccharide, a ketoheptose sugar such assedoheptulose may be utilized. Other stereoisomers of such carbohydratereactants not known to occur naturally are also contemplated to beuseful in preparing the coating compositions as described herein. When apolysaccharide serves as the carbohydrate, or is used in combinationwith monosaccharides, sucrose, lactose, maltose, starch, and cellulosemay be utilized.

Furthermore, the carbohydrate reactant in the Maillard reaction may beused in combination with a non-carbohydrate polyhydroxy reactant.Examples of non-carbohydrate polyhydroxy reactants which can be used incombination with the carbohydrate reactant include, but are not limitedto, trimethylolpropane, glycerol, pentaerythritol, polyvinyl alcohol,partially hydrolyzed polyvinyl acetate, fully hydrolyzed polyvinylacetate, and mixtures thereof. In one aspect, the non-carbohydratepolyhydroxy reactant is sufficiently nonvolatile to maximize its abilityto remain available for reaction with a monomeric or polymericcarboxylic acid reactant. It is appreciated that the hydrophobicity ofthe non-carbohydrate polyhydroxy reactant may be a factor in determiningthe physical properties of a coating prepared as described herein.

When a partially hydrolyzed polyvinyl acetate serves as anon-carbohydrate polyhydroxy reactant, a commercially available compoundsuch as an 87-89% hydrolyzed polyvinyl acetate may be utilized, such as,DuPont ELVANOL 51-05. DuPont ELVANOL 51-05 has a molecular weight ofabout 22,000-26,000 Da and a viscosity of about 5.0-6.0 centipoises.Other partially hydrolyzed polyvinyl acetates contemplated to be usefulin preparing coating compositions as described herein include, but arenot limited to, 87-89% hydrolyzed polyvinyl acetates differing inmolecular weight and viscosity from ELVANOL 51-05, such as, for example,DuPont ELVANOL 51-04, ELVANOL 51-08, ELVANOL 50-14, ELVANOL 52-22,ELVANOL 50-26, ELVANOL 50-42; and partially hydrolyzed polyvinylacetates differing in molecular weight, viscosity, and/or degree ofhydrolysis from ELVANOL 51-05, such as, DuPont ELVANOL 51-03 (86-89%hydrolyzed), ELVANOL 70-14 (95.0-97.0% hydrolyzed), ELVANOL 70-27(95.5-96.5% hydrolyzed), ELVANOL 60-30 (90-93% hydrolyzed). Otherpartially hydrolyzed polyvinyl acetates contemplated to be useful inpreparing coating compositions as described herein include, but are notlimited to, Clariant MOWIOL 15-79, MOWIOL 3-83, MOWIOL 4-88, MOWIOL5-88, MOWIOL 8-88, MOWIOL 18-88, MOWIOL 23-88, MOWIOL 26-88, MOWIOL40-88, MOWIOL 47-88, and MOWIOL 30-92, as well as Celanese CELVOL 203,CELVOL 205, CELVOL 502, CELVOL 504, CELVOL 513, CELVOL 523, CELVOL523TV, CELVOL 530, CELVOL 540, CELVOL 540TV, CELVOL 418, CELVOL 425, andCELVOL 443. Also contemplated to be useful are similar or analogouspartially hydrolyzed polyvinyl acetates available from other commercialsuppliers.

When a fully hydrolyzed polyvinyl acetate serves as a non-carbohydratepolyhydroxy reactant, Clariant MOWIOL 4-98, having a molecular weight ofabout 27,000 Da, may be utilized. Other fully hydrolyzed polyvinylacetates contemplated to be useful include, but are not limited to,DuPont ELVANOL 70-03 (98.0-98.8% hydrolyzed), ELVANOL 70-04 (98.0-98.8%hydrolyzed), ELVANOL 70-06 (98.5-99.2% hydrolyzed), ELVANOL 90-50(99.0-99.8% hydrolyzed), ELVANOL 70-20 (98.5-99.2% hydrolyzed), ELVANOL70-30 (98.5-99.2% hydrolyzed), ELVANOL 71-30 (99.0-99.8% hydrolyzed),ELVANOL 70-62 (98.4-99.8% hydrolyzed), ELVANOL 70-63 (98.5-99.2%hydrolyzed), ELVANOL 70-75 (98.5-99.2% hydrolyzed), Clariant MOWIOL3-98, MOWIOL 6-98, MOWIOL 10-98, MOWIOL 20-98, MOWIOL 56-98, MOWIOL28-99, and Celanese CELVOL 103, CELVOL 107, CELVOL 305, CELVOL 310,CELVOL 325, CELVOL 325LA, and CELVOL 350, as well as similar oranalogous fully hydrolyzed polyvinyl acetates from other commercialsuppliers.

The aforementioned Maillard reactants may be combined to make an aqueouscomposition that includes a carbohydrate reactant and an amine reactant.These aqueous compositions represent examples of uncured coatings. Asdiscussed below, these aqueous compositions can be used to form outercoatings on a proppant. These coatings are formaldehyde-free, curable,alkaline, aqueous coating compositions. Furthermore, as indicated above,the carbohydrate reactant of the Maillard reactants may be used incombination with a non-carbohydrate polyhydroxy reactant. Accordingly,any time the carbohydrate reactant is mentioned it should be understoodthat it can be used in combination with a non-carbohydrate polyhydroxyreactant.

In one illustrative embodiment, the aqueous solution of Maillardreactants may include (i) an ammonium salt of one or more carboxylicacid reactants and (ii) one or more carbohydrate reactants having areducing sugar. The pH of this solution prior to placing it in contactwith the material to be bound may be greater than or equal to about 7.In addition, this solution may have a pH of less than or equal to about10. The ratio of the number of moles of the carboxylic acid reactant(s)to the number of moles of the carbohydrate reactant(s) may be in therange from about 1:4 to about 1:15. In one example, the ratio of thenumber of moles of the carboxylic acid reactant(s) to the number ofmoles of the carbohydrate reactant(s) in the coating composition may beabout 1:5. In another example, the ratio of the number of moles of thecarboxylic acid reactant(s) to the number of moles of the carbohydratereactant(s) may be about 1:6. In yet another example, the ratio of thenumber of moles of the carboxylic acid reactant(s) to the number ofmoles of the carbohydrate reactant(s) may be about 1:7.

As described above, the aqueous coating composition may include (i) anammonium salt of one or more carboxylic acid reactants and (ii) one ormore carbohydrate reactants having a reducing sugar. It should beappreciated that when an ammonium salt of a monomeric or a polymericcarboxylic acid is used as an amine reactant, the molar equivalents ofammonium ion may or may not be equal to the molar equivalents of acidsalt groups present on the carboxylic acid. In one illustrative example,an ammonium salt may be monobasic, dibasic, or tribasic when atricarboxylic acid is used as a polycarboxylic acid reactant. Thus, themolar equivalents of the ammonium ion may be present in an amount lessthan or about equal to the molar equivalents of acid salt groups presentin a carboxylic acid. Accordingly, the salt can be monobasic or dibasicwhen the polycarboxylic acid reactant is a dicarboxylic acid. Further,the molar equivalents of ammonium ion may be present in an amount lessthan, or about equal to, the molar equivalents of acid salt groupspresent in a polymeric polycarboxylic acid, and so on and so forth. Whena monobasic salt of a dicarboxylic acid is used, or when a dibasic saltof a tricarboxylic acid is used, or when the molar equivalents ofammonium ions are present in an amount less than the molar equivalentsof acid salt groups present in a polymeric polycarboxylic acid, the pHof the coating composition may require adjustment to achieve alkalinity.

Curing of the outer coating may take place by heating the coatedsubstrate to a sufficient curing temperature for a time sufficient toachieve substantial curing of the coating. The curing temperature maybe, for example, from about 300° F. (149° C.) to about 600° F. (316°C.).

With respect to making coatings that are water-insoluble when cured, itshould be appreciated that the ratio of the number of molar equivalentsof acid salt groups present on the carboxylic acid reactant(s) to thenumber of molar equivalents of hydroxyl groups present on thecarbohydrate reactant(s) may be in the range from about 0.04:1 to about0.15:1. After curing, these formulations result in a water-resistantthermoset coating. In one variation, the number of molar equivalents ofhydroxyl groups present on the carbohydrate reactant(s) is about twentyfive-fold greater than the number of molar equivalents of acid saltgroups present on the carboxylic acid reactant(s). In another variation,the number of molar equivalents of hydroxyl groups present on thecarbohydrate reactant(s) is about ten-fold greater than the number ofmolar equivalents of acid salt groups present on the carboxylic acidreactant(s). In yet another variation, the number of molar equivalentsof hydroxyl groups present on the carbohydrate reactant(s) is aboutsix-fold greater than the number of molar equivalents of acid saltgroups present on the carboxylic acid reactant(s).

Various additives can be incorporated into the coating compositions.These additives give the coatings of the present invention additionaldesirable characteristics. For example, the coating may include asilicon-containing coupling agent, especially when the coating is usedas an innermost of a substrate including a silica containing material,such as sand. Many silicon-containing coupling agents are commerciallyavailable from the Dow-Corning Corporation, Petrarch Systems, and by theGeneral Electric Company. Illustratively, the silicon-containingcoupling agent includes compounds such as silylethers and alkylsilylethers, each of which may be optionally substituted, such as withhalogen, alkoxy, amino, and the like. In one variation, thesilicon-containing compound is an amino-substituted silane, such as,gamma-aminopropyltriethoxy silane (General Electric Silicones, SILQUESTA-1101; Wilton, Conn.; USA). In another variation, thesilicon-containing compound is an amino-substituted silane, for example,aminoethylaminopropyltrimethoxy silane (Dow Z-6020; Dow Chemical,Midland, Mich.; USA). In another variation, the silicon-containingcompound is gamma-glycidoxypropyltrimethoxysilane (General ElectricSilicones, SILQUEST A-187). In yet another variation, thesilicon-containing compound is an n-propylamine silane (Creanova(formerly Huls America) HYDROSIL 2627; Creanova; Somerset, N.J.;U.S.A.).

The silicon-containing coupling agents may be present in the coating inthe range from about 0.1 percent to about 1 percent by weight based uponthe dissolved coating solids (i.e., about 0.1 percent to about 1 percentbased upon the weight of the solids added to the aqueous solution). Inone application, one or more of these silicon-containing compounds canbe added to the aqueous uncured coating. The coating is then applied tothe material to be coated. Thereafter, the coating may be cured orpartially cured.

A coating that includes a silicone containing coupling agent can beprepared by admixing about 10 to about 50 weight percent aqueoussolution of one or more carboxylic acid reactants, already neutralizedwith an amine base or neutralized in situ, with about 10-50 weightpercent aqueous solution of one or more carbohydrate reactants havingreducing sugar, and an effective amount of a silicon-containing couplingagent. In one variation, one or more carboxylic acid reactants and oneor more carbohydrate reactants, the latter having reducing sugar, may becombined as solids, mixed with water, and the mixture then treated withaqueous amine base (to neutralize the one or more carboxylic acidreactants) and a silicon-containing coupling agent to generate anaqueous solution 10-50 weight percent in each carboxylic acid reactantand each carbohydrate reactant.

By following guidelines disclosed herein, one of ordinary skill in theart will be able to vary the concentrations of the reactants of theaqueous coating to produce a wide range of coating compositions. Inparticular, aqueous coating compositions can be formulated to have analkaline pH. For example, a pH in the range from greater than or equalto about 7 to less than or equal to about 10. Examples of the coatingreactants that can be manipulated include (i) the carboxylic acidreactant(s), (ii) the amine base, (iii) the carbohydrate reactant(s),and (iv) the silicon-containing coupling agent. Having the pH of theaqueous coatings (e.g., uncured coatings) in the alkaline range inhibitsthe corrosion of materials the coating comes in contact with.

Example 1 Preparation of Outer Coating Resin

Citric acid, dextrose and ammonium hydroxide were used to form a coatingresin. 23.9 grams of anhydrous citric acid and 143.2 grams of dextrosewere added to 200 grams of deionized water which was at roomtemperature, 22° C. The solution was mixed for ten minutes and then 32.2grams of 19% ammonium hydroxide solution was added. Mixing continued for25 minutes and then 1.2 grams of SILQUEST A1100 silane was added. Mixingwas continued for another 10 minutes, and the liquid resin was thenbottled. The liquid resin had approximately 41.7% solids (based onweight of dry adds), a pH of 6.10 and a cure at 150° C. on a hot platefor four minutes and 30 seconds.

Example 2 Coating of a Cured Proppant

The liquid resin from Example 1 was used to produce a cured single layercoated proppant. 3,000 grams of 20/40 grade frac sand from US Silica waspreheated in a 500° F. (260° C.) forced air oven. The heated sand wasplaced in a laboratory scale pug mill. After ten seconds of mixing thetemperature of the frac sand was 472° F. (244° C.) as measured with aninfared temperature gun. 120 grams of the polymer from Example 1 wasadded. Mixing continued for one minute and fifty seconds. The coatedfrac sand was discharged from the pug mill, screened to remove anyagglomerations and air cooled by pouring from pan to pan several times.The coated frac sand was a dark brown in color and was free flowing.

The coated sand of Example 2 was tested for curabilty as measured aDieter 365A Hot Shell Tensile Tester. Curing the coated sand for threeminutes at 450° F. (232° C.) and then pulling the dog bone gave astrength of 0 psi. Another sample of the coated sand from Example 2 wasplaced in a three cavity dog bone maker. The coated frac sand wasoverfilled and leveled to give approximately a quarter of an inch excessabove the surface of the dog bone form. The dog bone form was sandwichedbetween metal plates and the entire assembly was placed into a heatedpress. The platens of the press were at 450° F. (232° C.) and pressurewas applied to give an effective 2,000 psi (13,800 kPa) on the dog bonemaker, and heat and pressure was applied for ten minutes. Following thatthe assembly was removed from the press and allowed to cool to roomtemperature. On removal of the metal plates the coated frac sand wasfree flowing indicating no bonding under heat and pressure.

Example 3 Preparation of a Multi Coated Curable Proppant

3,000 grams of 20/40 grade frac sand from US Silica was preheated in a500° F. (260° C.) forced air oven. The heated sand was placed in alaboratory scale pug mill. After fifteen seconds of mixing thetemperature of the frac sand was 420° F. (216° C.) as measured with aninfared temperature gun. 60 grams of GP-2202, a commericaly availableresin from Georiga-Pacific Chemicals LLC, was added. Mixing wascontinued for fortyfive seconds. A solution of 6.6 grams ofhexamethylenetetramine in 30 mls of water was added. Mixing continuedfor another twenty seconds. At this point 84 grams of the polymer madein Example 1 was added. Mixing continued for another minute and fortyseconds. At this point the coated frac sand was discarged from the pugmill. The coated frac sand was a brownish golden yellow color. Asdischarged at 214° F. (101° C.) the mixture exhibited a tendency toblock, that is stick together, but on further cooling below 135° F. (57°C.), the coated sand remained free flowing.

The coated sand made in Example 3 was tested for hot tensile strength asdescribed in Example 2. After three minutes curing at 450° F. (232° C.),a bond strength of 40 psi (276 kPa) resulted.

Again another sample of the coated sand from Example 3 was overfilledinto a three position dog bone maker, leveled off and sandwiched betweentwo metal plates. The assembly was put into a heated press and allowedto cure under 2,000 psi (13,800 kPa) effective pressure and 450° F.(232° C.) for ten minutes. The assembly was removed and allowed to coolto room temperature. The formed dog bones were removed from the form andthe bond strengths determined by a Dieter 400A Universal Sand StrengthMachine. The average for the test was 340 psi (2,340 kPa).

The 40 psi (276 kPa) hot tensile test shows that the top layer of sugarresin is shielding the underlying novolac hexamethylenetetramine curablelayer. Thus, low bond strength under no pressure was obtained. The dogbones cured under heat and pressure show that the inner layer is stillcurable and the pressure allows this inner layer to ooze out, flow andto bond. This is shown by the high strength of 340 psi (2,340 kPa).

Example 4 Preparation of Outer Coating Resin

A resin was prepared substantially as in Example 1, except that a highersolids content was obtained. To 58 grams of deionized water at roomtemperature, 12.3 grams of citric acid and 73.6 grams of dextrose wereadded and mixed for 15 minutes. 16.6 grams of 19% ammonium hydroxide wasadded and mixed for 10 minutes. 0.62 Grams of SILQUEST A1100 silane wasadded and mixed for 10 more minutes and then the resin solution wasbottled. This resin solution had a pH of 5.95, and a RI of 1.4362. 28%ammonium hydroxide was added drop wise (0.67 grams of ammonia added) tothis solution while stirring to give a final solution of pH 7.85 and a150° C. Hot Plate Cure for four minutes and 10 seconds. The calculatedsolids content based on the weight of the dry adds was 53%.

Example 5 Coating of a Curable Proppant

The liquid resin solution from Example 4 was used to coat a proppant.3,000 grams of 20/40 frac sand, from US Silica, was preheated in a 500°F. (260° C.) electric oven.

The heated frac sand was put into a laboratory sized pug mill. Aftermixing five seconds, 60 grams of GP-2202, a commercially availty fracsand novoalc available from Georgia Pacific Chemicals LLC, was added tothe hot sand at a temperature of 480° F. (249° C.) as measured by aninfared temperature gun. Mixing was carried out for forty five seconds,at which time 6.6 grams of hexamethylenetetraime dissolved in 21 gramsof water was added. Mixing was continued for another ten seconds. Atthis time 85 grams of the polymer made in Example 4 was added. Thetemperature at the time of the polymer addition was 326° F. (163° C.).Mixing was continued for one minute and fifty five seconds. Thetemperare of the coated sand was 249° F. (121° C.). 67 grams of waterwas added to the coated sand and mxing coninuted for another minute andthirty seconds. At this time the coated sand was discharged from the pugmill at a temperature of 150° F. (66° C.). The coated sand exhibitedsome blockage and stick together, but as the mass cooled to 110° F. (43°C.), it then remained free flowing. The coated sand was brown in color.

The coated sand made in Example 5 was tested for bonding strengths. Thethree minute hot tensile test as measured by a Dieter 365A Hot ShellTensile Tester gave a strength of 0 psi (0 kPa). Another sample of thiscoated sand was put into a three position dog bone maker and put in aheated press for ten minutes. The temperature of the heated platens wasset at 450° F. (232° C.), and an effective overall pressure on the dogbone maker was 2,000 psi (13,800 kPa). At the end of the 10 minutes thedog bone maker was taken out of the press. The dog bones were separatedfrom the form and allowed to cool to room temperature. Their strengthwas measured on a Dieter 400-A Universal Sand Strength Machine. Thisgave an average strength of 150 psi (1,030 kPa).

The 0 psi (0 kPa) hot tensile shows that the top layer of sugar resinwas cured and shields the underlying novolac hexamethylenetetraminecurable layer. Thus, no bond strength was obtained while curing under nopressure. The dog bones cured under heat and pressure show that theinner layer is still curable and the pressure allows this inner layer toooze out, flow and to bond. This is shown by the high strength of 150psi (1,030 kPa). This run's strength is lower because the highertemperatures that the coating process was held at resulted in partialcuring of the underlying novolac hexamethylenetetramine layer. However,the main point demonstrated is that the partially cured inner layer isstill capable of further curing and, when fully cured, resulted in highbond strength.

Comparative Example

Coating with Sugar

In this example dextrose was used as the outer curable coating. Thecoated proppant upon coating was gummy and remained gummy after coolingto room temperature and would not be suitable for use. This shows thatsugar by itself is not effective.

Formation Treatment

The free-flowing particles as produced by the above method may be usedas proppants, gravel or fluid loss agents in hydraulic fracturing, fracpacking and gravel packs. In carrying out a hydraulic fracturingoperating a fracture is first generated by injecting a viscous fluidinto the formation at a sufficient rate and pressure to cause theformation to fail in tension. Injection of the fluid is typicallycontinued until a fracture of the desired geometry is obtained. Acarrier fluid having the proppant suspended therein is then pumped intothe fracture. The carrier fluid bleeds off into the formation anddeposits the propping agent in the fracture. This process is controlledby fluid loss agents which are small aggregate particles whichtemporarily slow the fluid loss to the formation.

After the proppant is placed, the well is shut in with pressuremaintained on the formation. As the pressure within the fractureapproaches the normal formation pressure, the fracture walls close in onthe proppant and apply an overburden stress thereto. When the proppantincludes a cured outer coating and a curable inner coating, it isbelieved this stress ruptures the outer coating exposing the curableinner coating. At the same time ambient formation temperature heats theinner resin coating. When the inner resin coating includes athermosetting resin, the resin fuses and unites at contact areas betweencontiguous particles or with the formation walls. As the temperatureincreases the polymerization reaction proceeds until the resin is curedinto an insoluble and infusible cross-linked state. The pendular regionsbetween adjacent particles bond the packed particles into a permeablemass having considerable compressive strength.

A more detailed description of the standard industry practices for theuse of resin coated particles in hydraulic fracturing and gravel packcompletion is disclosed in U.S. Pat. No. 3,929,191. Details of suchapplications are well known in the art.

In another embodiment, the present invention is:

-   -   1. A proppant for a well treatment fluid, the proppant        comprising discrete particles of a substrate coated with a resin        comprising a product of the Maillard reaction between a        carbohydrate and an amine and/or an ammonium compound.    -   2. The proppant of any of the preceding paragraphs, wherein the        carbohydrate includes a monosaccharide.    -   3. The proppant of any of the preceding paragraphs, wherein the        monosaccharide includes a reducing sugar.    -   4. The proppant of any of the preceding paragraphs, wherein the        reducing sugar includes at least one of dextrose, xylose,        fructose, and dihydroxyacetone.    -   5. The proppant of any of the preceding paragraphs, wherein said        reaction is between a carbohydrate and an ammonium salt of a        carboxylic acid.    -   6. The proppant of any of the preceding paragraphs, wherein said        reaction is between a carbohydrate and an ammonium salt of a        polycarboxylic acid.    -   7. The proppant of any of the preceding paragraphs, wherein said        polycarboxylic acid comprises citric acid.    -   8. The proppant of any of the preceding paragraphs, wherein the        reaction mixture for forming said resin comprises a polyol.    -   9. The proppant of any of the preceding paragraphs, wherein the        reaction mixture for forming said resin comprises a        phosphorous-containing accelerator.    -   10. The proppant of any of the preceding paragraphs, wherein the        substrate is coated with a single layer of a coating comprising        the Maillard reaction product.    -   11. The proppant of any of the preceding paragraphs, wherein the        substrate is coated with two layers of coating, wherein a first,        innermost layer comprises a fusible resin, and a second,        outermost layer comprises said product of said Maillard        reaction.    -   12. The proppant of any of the preceding paragraphs, wherein        said fusible resin is a thermoplastic resin.    -   13. The proppant of any of the preceding paragraphs, wherein the        thermoplastic resin is comprised of an ethylene vinyl acetate, a        phenol-formaldehyde novolac resin, a pine rosin, a modified        rosin, a rosin ester, a terpene resin, a coumarone-indene resin,        an oligomer of C₅ hydrocarbons, an oligomer of C₉ hydrocarbons,        an oligomeric reaction product of a terpene and a phenolic, and        an oligomeric reaction product of a terpene and a styrenic, or        any combination thereof    -   14. The proppant of any of the preceding paragraphs, wherein        said fusible resin is a curable, thermosetting resin.    -   15. The proppant of any of the preceding paragraphs, wherein        said curable, thermosetting resin is selected from        phenol-aldehyde resins, urea-aldehyde resins, melamine-aldehyde        resins, epoxy resins, furfuryl alcohol resins, and copolymers of        such resins.    -   16. The proppant of any of the preceding paragraphs, wherein the        substrate particles comprise at least one of sand, glass beads,        ceramics, carbon particles, and bauxite.    -   17. The proppant of any of the preceding paragraphs, wherein the        substrate particles comprise as least one of crushed nut hulls        and other natural fibers.    -   18. A fracturing fluid for stimulating hydrocarbon production        from a subterranean formation, the fracturing fluid comprising a        suspension of a carrier liquid and a proppant comprising        discrete particles of a substrate coated with a resin comprising        a product of the Maillard reaction between a carbohydrate and an        amine or an ammonium compound.    -   19. The fracturing fluid of any of the preceding paragraphs,        wherein the carbohydrate includes a monosaccharide.    -   20. The fracturing fluid of any of the preceding paragraphs,        wherein the monosaccharide includes a reducing sugar.    -   21. The fracturing fluid of any of the preceding paragraphs,        wherein the reducing sugar includes at least one of dextrose,        xylose, fructose, and dihydroxyacetone.    -   22. The fracturing fluid of any of the preceding paragraphs,        wherein said reaction is between a carbohydrate and an ammonium        salt of a carboxylic acid.    -   23. The fracturing fluid of any of the preceding paragraphs,        wherein said reaction is between a carbohydrate and an ammonium        salt of a polycarboxylic acid.    -   24. The fracturing fluid of any of the preceding paragraphs,        wherein said polycarboxylic acid comprises citric acid.    -   25. The fracturing fluid of any of the preceding paragraphs,        wherein the substrate is coated with two layers of coating,        wherein a first, innermost layer comprises a fusible resin, and        a second, outermost layer comprises said product of said        Maillard reaction.    -   26. The fracturing fluid of any of the preceding paragraphs,        wherein the layer of fusible resin is composed of a curable,        thermosetting resin selected from phenol-aldehyde resins,        urea-aldehyde resins, melamine-aldehyde resins, epoxy resins,        furfuryl alcohol resins, and copolymers of such resins.    -   27. The fracturing fluid of any of the preceding paragraphs,        wherein the substrate particles comprise at least one of sand,        glass beads, ceramics, carbon particles, and bauxite.    -   28. A process for stimulating hydrocarbon production from a        subterranean formation, the process comprising injecting into        said formation a fracturing fluid comprising a suspension of a        carrier liquid and a proppant comprising discrete particles of a        substrate coated with a cured resin comprising a product of the        Maillard reaction between a carbohydrate and an amine or an        ammonium compound.    -   29. The process of any of the preceding paragraphs, wherein the        carbohydrate includes a monosaccharide.    -   30. The process of any of the preceding paragraphs, wherein the        monosaccharide includes a reducing sugar.    -   31. The process of any of the preceding paragraphs, wherein the        reducing sugar includes at least one of dextrose, xylose,        fructose, and dihydroxyacetone.    -   32. The process of any of the preceding paragraphs, wherein said        reaction is between a carbohydrate and an ammonium salt of a        carboxylic acid.    -   33. The process of any of the preceding paragraphs, wherein said        reaction is between a carbohydrate and an ammonium salt of a        polycarboxylic acid.    -   34. The process of any of the preceding paragraphs, wherein said        polycarboxylic acid comprises citric acid.    -   35. The process of any of the preceding paragraphs, wherein the        substrate is coated with two layers of coating, wherein a first,        innermost layer comprises a fusible resin, and a second,        outermost layer comprises said product of said Maillard        reaction.    -   36. The process of any of the preceding paragraphs, wherein said        fusible resin is selected from phenol-aldehyde resins,        urea-aldehyde resins, melamine-aldehyde resins, epoxy resins,        furfuryl alcohol resins, and copolymers of such resins.    -   37. The process of any of the preceding paragraphs, wherein the        substrate particles comprise at least one of sand, glass beads,        ceramics, carbon particles and bauxite.

Further modifications and alternate embodiments of the invention will beapparent to those skilled in the art in view of this description.Accordingly, this description is to be considered as illustrative onlyand for the purpose of teaching those skilled in the art the manner ofcarrying out the invention. Various modifications may be made in themethod. Applicants intend that all such modifications, alterations andvariations which fall within the spirit and scope of the appended claimsbe embraced thereby.

What is claimed is:
 1. A method for making a proppant for a welltreatment fluid, comprising: coating a particle with a mixture ofMaillard reactants selected from the group consisting of: (1) acarbohydrate and an amine, (2) a carbohydrate and an ammonium compound,and (3) a carbohydrate, an amine, and an ammonium compound; and heatingthe particle coated with the mixture of Maillard reactants to atemperature of about 149° C. to about 316° C. to produce the proppant.2. The method of claim 1, wherein the particle comprises sand, glass,ceramic, carbon, or bauxite.
 3. The method of claim 1, wherein theparticle comprises sand.
 4. The method of claim 1, wherein heating theparticle coated with the mixture of Maillard reactants forms awater-insoluble coating of a Maillard reaction product on the particle.5. The method of claim 1, wherein the mixture of Maillard reactantscomprises the carbohydrate and the ammonium compound, and wherein theammonium compound comprises an ammonium salt of a carboxylic acid. 6.The method of claim 1, wherein the mixture of Maillard reactantscomprises the carbohydrate and the ammonium compound, and wherein theammonium compound comprises an ammonium salt of a polycarboxylic acid.7. The method of claim 1, wherein: the carbohydrate comprises amonosaccharide, heating the particle coated with the mixture of Maillardreactants forms a Maillard reaction product, and the particle is coatedwith a single layer of a coating comprising the Maillard reactionproduct.
 8. The method of claim 1, wherein the carbohydrate comprisesdextrose, xylose, fructose, dihydroxyacetone, or any mixture thereof. 9.The method of claim 1, wherein: heating the particle coated with themixture of Maillard reactants forms a Maillard reaction product, theparticle is coated with an inner layer and an outer layer, the innerlayer comprises a curable thermosetting resin, and the outer layercomprises the Maillard reaction product.
 10. The method of claim 1,wherein the mixture of Maillard reactants further comprises an additive,and wherein the additive comprises a silicone containing coupling agent.11. The method of claim 1, wherein the mixture of Maillard reactantsfurther comprises a silane coupling agent, and wherein the silanecoupling agent comprises a gamma-aminopropyltriethoxysilane, anaminoethylaminopropyltrimethoxysilane, agamma-glycidoxypropyltrimethoxysilane, or an n-propylamine silane. 12.The method of claim 1, wherein: the particle comprises sand or ceramic,the mixture of Maillard reactants comprises the carbohydrate and theammonium compound, the carbohydrate comprises a monosaccharide, and theammonium compound comprises an ammonium salt of a carboxylic acid. 13.The method of claim 12, wherein the mixture of Maillard reactantsfurther comprises an additive, and wherein the additive comprises asilicone containing coupling agent.
 14. A method for making a proppantfor a well treatment fluid, comprising: coating particles of sand with amixture of Maillard reactants selected from the group consisting of: (1)a monosaccharide and an amine, (2) a monosaccharide and an ammoniumcompound, and (3) a monosaccharide, an amine, and an ammonium compound;and heating the particles of sand coated with the mixture of Maillardreactants to a temperature of about 149° C. to about 316° C. to producethe proppant.
 15. The method of claim 14, wherein less than 5 wt % ofthe proppant is agglomerated.
 16. The method of claim 14, wherein theparticles of sand coated with the mixture of Maillard reactants compriseabout 1 wt % to about 6 wt % of the mixture of Maillard reactants, basedon the weight of the particles of sand.
 17. The method of claim 14,wherein the mixture of Maillard reactants comprises the monosaccharideand the ammonium compound, and wherein the ammonium compound comprisesan ammonium salt of a carboxylic acid.
 18. The method of claim 14,wherein: heating the particle coated with the mixture of Maillardreactants forms a water-insoluble coating of a Maillard reaction producton the particles of sand, less than 5 wt % of the proppant isagglomerated, the particles of sand coated with the mixture of Maillardreactants comprise about 1 wt % to about 6 wt % of the mixture ofMaillard reactants, based on the weight of the particles of sand, themixture of Maillard reactants comprises the monosaccharide and theammonium compound, and the ammonium compound comprises an ammonium saltof a polycarboxylic acid.
 19. A method for making a proppant for a welltreatment fluid, comprising: coating a plurality of particles with amixture of Maillard reactants comprising a carbohydrate and an ammoniumsalt of a carboxylic acid, wherein the plurality of particles comprisesand, glass, ceramic, carbon, bauxite, or any mixture thereof; andheating the plurality of particles coated with the mixture of Maillardreactants to a temperature of about 149° C. to about 316° C. to producethe proppant.
 20. The method of claim 19, wherein: the carbohydratecomprises dextrose, xylose, fructose, dihydroxyacetone, or any mixturethereof, the ammonium salt of the carboxylic acid comprises an ammoniumsalt of citric acid, the plurality of particles coated with the mixtureof Maillard reactants comprises about 1 wt % to about 6 wt % of themixture of Maillard reactants, based on the weight of the plurality ofparticles, heating the particle coated with the mixture of Maillardreactants forms a water-insoluble coating of a Maillard reaction producton the plurality of particles, the mixture of Maillard reactants furthercomprises a silicone containing coupling agent, and less than 5 wt % ofthe proppant is agglomerated.